Pencil leads are essentially baked ceramic rod of clay bonded graphite. The suitability of graphite for pencil industry is judged by the dark streak it leaves on the paper by scratching. Synthetic graphite though has less ash content and very fine particle size, produces very little smear and thus, is unsuitable for pencil manufacturers. Usually pencil manufacturers prefer amorphous graphite with about 90% purity and free from gritty particles. Amorphous graphite gives better smear than the flaky graphite. Indian Standard Institute (ISI) has thus set norm for graphite suitable for pencil industry as—amorphous graphite with size −300 BS mesh and ash less than 50%. Micronised amorphous graphite accordingly is the very much sought after raw material for the manufacture of high class lead for pencils.
General procedure followed for making pencil lead for these micronised amorphous graphite involves, ball-milling or hammer-milling a water slurry containing clay and graphite, drying the slurry to required consistency and form a stiff dough in a mixer, followed by compacting it into a cylinder to form and extruding under pressure through a die. The wet strands are then dried, packed in saggers and kiln fired in a temperature range of 800–1100° C. The fired leads are then impregnated with waxes and fats or fatty-acids, or both. This latter step preclude glazing of the point of use. The quality of the lead depends on the quality of the ingredients used and the manufacturing process adopted. The degree of hardness depends on the ratio of clay to graphite. Clay percentage used varies from 20–60%. Pencils marked No. 1 (softest) lead contains about 20% clay while No. 4 (hardest) has about 60% clay. Hardness is a product requirement and not a quality factor such as the uniformity, smoothness or strength of the sharpened point. Indelible leads, on the other hand, are mixtures of graphite, methyl violet and binders such as gum-tragacanth or methyl-cellulose, with or without mineral fillers and insoluble soaps. Although above account exemplifies traditional and time tested industrially proven receipt and process for the production of pencil leads, a great variety of ingredients and processes have been reported in recent times.